(1) Field of the Invention
The present invention relates to tires for use in general vehicles, particularly trucks, buses and modern urban traffic vehicles such as monorail cars, subway trains using rubber tires and the like. More particularly, the invention relates to a low noise level tire which lowers both passing noises and acceleration noises generated from the tire to reduce vehicle-outside noise including the noises of these types, and simultaneously maintains its wear-resistance as an essential performance at a high level as desired.
(2) Related Art Statement
Recently, great interest has risen in the noise dispersed from vehicles among various factors of tires affecting the social environment, and demand for the reduction of the noise generated from the tires themselves has been increasing, in addition to the economical improvement of tires, which is a fundamental requirement. Such demand for the reduction of noise has risen with respect to not only tires for cars running on general roads but also tires for monorail cars, subway trains and other modern urban traffic vehicles.
In the past, noise generated from the tire to the outside of a test vehicle (vehicle-outside noise) was usually measured in a state where the vehicle was drifted after a clutch in the vehicle had been disengaged. A microphone was fixedly arranged at a position spaced 7.5 m from a running way of the tire to be tested, and the noise was measured when the vehicle passed by the position. The magnitude of the passing noise was judged by the measured results. However, since it is considered that the passing noise is not sufficient to represent the actual vehicle noise, a conclusion has been reached that a combination of the acceleration noise and the passing noise must be considered with the same importance for estimating the actual vehicle noise.
In order to lower the passing noise in the reduction of the tire noise, it has been proposed to modify grooves formed in treads or to change the configuration of treads.
Moreover, for example, Japanese Patent Application Laid-open No. 2-225106 disclosed a tire whose tread is composed of a usual cap rubber layer and a base layer made of a foam rubber having a low elastic modulus and a high loss tangent in order to maintain wear-resistance and improve the riding comfortability against vibration, although it does not aim to directly reduce the vehicle-outside noise.
Furthermore, Japanese Patent Application Laid-open No. 2-60803 disclosed a tire adapted to reduce the noise in a vehicle caused by vibration when running on a bad road without lowering the cornering performance, although it does not aim to directly reduce the vehicle-outside noise, either. The disclosed tire has a tread including a cap layer and an inner layer. The cap layer is made of a usual rubber having a high hardness and a high elastic modulus, and the inner layer is made of the same rubber as that of the cap layer but mixed with 3% to 10% by volume of a vulcanized rubber particles having a low hardness to have a low elastic modulus and a low loss tangent.
Among the proposals described above, with the modified grooves in the treads or the modified treads themselves, the wear-resistance is unavoidably lowered and further reduction in acceleration noise is scarcely possible because no attention is paid to the reduction in the acceleration noise, although the passing noise can be reduced.
The techniques disclosed in Japanese Patent Application Laid-open Nos. 2-225106 and 2-60803 have suggested the reduction in the vehicle-outside noise. However, with the tire disclosed in the Japanese Patent Application Laid-open No. 2-225106, the acceleration noise is rather increased according to results of experiments carried out by the present inventors, although both wear-resistance and the riding comfortability against vibration can be simultaneously improved and the passing noise can be lowered, the acceleration noise inversely becomes greater.
Moreover, with the tire disclosed in Japanese Patent Application Laid-open No. 2-60803, the passing noise greatly increases with respect to the vehicle-outside noise, although the acceleration noise can be reduced.
By way of experiment, tires were produced whose cap layer of the tread was made of a rubber having a low elastic modulus and a high loss tangent within a range guessed at from the disclosures of the above two Japanese Applications, and these tires were tested. As a result, the passing and acceleration noises were reduced, but the wear-resistance was lowered to an extent that the tires could not be practically used.
In measuring the passing and acceleration noise in the present invention, a microphone is arranged adjacent a tire so as to measure only the noise generated from the tire in order to exclude the influence of background noise as much as possible. This is different from the conventional measurement made by arranging a microphone at a position remote from the vehicle as described above.
From the above explanation, it is concluded that there is no prior art technique which can reduce the passing and acceleration noise simultaneously without lowering the wear-resistance of a tire.